This invention relates to a receiver for a digital audio broadcast.
As a digital audio broadcast, DAB (Digital Audio Broadcasting) which complies with the Eureka 147 standards is carried out in Europe. The DAB prepares a function that a broadcasting station controls the gain of an audio signal line of a DAB receiver.
This function is called DRC (Dynamic Range Control). In order to realize the DRC, a broadcasting station adds control data called DRC data to original digital audio data and sends out resulting data.
FIG. 4A shows a structure of DAB data in one service transmitted by the DAB. Referring to FIG. 4A, the DAB data is formed by a plurality of frames. A header is disposed at the top of each frame, and audio data and data of a SCF (SCale Factor) and so forth are disposed following the header. The audio data is data (hereinafter referred to as MPEG audio data) obtained by compressing original linear digital audio data in accordance with the layer II of the MPEG audio standards, and the SCF is a scale factor used upon such data compression.
Further, an area called PAD (Program Associated Data) is prepared at the last of the frame, and DRC data is disposed at part of the PAD. In this instance, however, if the arbitrary frame is the nth frame, then the DRC data of the nth frame is defined as being effective on digital audio data on the next (n+1)th frame.
For example, in the mode II, the frame period is 24 milliseconds, and accordingly, also DRC data is sent out after each 24 milliseconds. Further, the DRC data and the gain of the audio signal line have the following relationship:
Accordingly, if, for example, xe2x80x9c000011xe2x80x9d is sent as DRC data from a broadcasting station, then the gain of the audio signal line of a DAB receiver increases by 0.75 dB, and as a result, the volume of sound of the DAB receiver becomes greater by 0.75 dB than a sound volume set by its user.
Accordingly, for example, if the sound volume of the DAB receiver is controlled as in AGC with DRC data while a news program is on the air, then the articulation of sound received can be raised. Or in the case of urgent broadcasting, the sound volume of the DAB receiver can be increased compulsorily so that the listener can hear the urgent broadcast with certainty.
However, development of a DAB receiver conducted with a conventional technique has revealed that a displacement in time occurs in gain control with DRC data.
In particular, since, in the DAB, digital audio data and DRC data are processed in a unit of a frame, when a DAB receiver extracts MPEG audio data from DAB data of FIG. 4A and decodes them into digital audio data, the digital audio data is obtained, for example, after 8 milliseconds from the starting point of time of a next frame as seen from FIG. 4B.
Further, when data of a PAD of the DAB data of FIG. 4A is analyzed to extract DRC data, the DRC data is obtained, for example, after 22.8 milliseconds from the starting point of time of a next frame as seen also from FIG. 4C.
The DAB receiver multiplies the digital audio data by the DRC data to perform gain control. Thus, before the DRC data is set to the multiplication circuit, a time of, for example, 2.2 milliseconds is required as seen in FIG. 4D.
Accordingly, where the numerical value examples given above apply, a time difference of, for example, 7 milliseconds appears between the digital audio data and the DRC data set to the multiplication circuit as seen from FIGS. 4B and 4D. In this instance, since the DRC data of a certain frame corresponds to the digital audio data of the next frame, the sound volume control with the DRC data is executed earlier by, for example, 7 milliseconds than the original time.
The time difference between the digital audio data and the DRC data is different depending upon the design of the circuit, and in some cases, the digital audio data may possibly be obtained later than the DRC data.
The time difference between the digital audio data and the DRC data can be corrected by calculating times required for signal processing of the individual data and delaying one of the data in accordance with the calculated values. However, since the signal processing for the individual data is complicated, the time difference cannot be discriminated accurately.
Accordingly, a time difference appears between the digital audio data and the DRC data as described above, and sound volume control with the DRC data is displaced in time.
It is an object of the present invention to provide a receiver for a digital audio signal by which the timings of digital audio data and DRC data in a DAB receiver can be adjusted to prevent displacement in time of sound volume control with the DRC data.
In order to attain the object described above, according to the present invention, there is provided a receiver for a digital audio broadcast for receiving a digital audio broadcast by which digital audio data and control data incidental to the digital audio data are sent out, comprising a reception circuit for receiving the digital audio broadcast, a decoder circuit for extracting the digital audio data and the control data from a signal received by the reception circuit, a D/A converter circuit for performing D/A conversion of the digital audio data extracted by the decoder circuit into an analog audio signal and outputting the analog audio signal, control means for controlling a characteristic of the analog audio signal in accordance with the control data extracted by the decoder circuit, and a memory into which that one of the digital audio data and the control data which has been extracted at an earlier timing by the decoder circuit is written, the control means successively varying a write address for the memory in a sampling period of the digital audio data while the control means successively reading out the data written in the memory with a read address having a predetermined address difference from the write address and uses the data thus read out to execute control of the characteristic of the analog audio signal with the control data extracted by the decoder circuit.
In the receiver for a digital audio broadcast, the time difference between digital audio data and control data for the digital audio data can be corrected with the memory. In this instance, even if signal processing for the digital audio data and the DRC data is complicated and the time required for the signal processing cannot be calculated accurately, the time difference between the digital audio data and the DRC data can be corrected accurately.
Besides, the time difference between the digital audio data and the DRC data can be corrected with a resolution of one sample period of the digital audio data. Further, even if the digital audio data have a plurality of sampling periods, the receiver for a digital audio broadcast can cope with this simply. Furthermore, whichever one of timings of the digital audio data and the control data is delayed, the receiver for a digital audio broadcast can cope with this.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.